Pulping wood chips with peracetic acid and chlorine dioxide



United States Patent 3,458,394 PULPING W001) CHIPS WITH PERACETIC ACID AND CHLORINE DIOXIDE Peter N. Yiannos, Wiirnington, Del., and Kenneth W. Britt, Norwood, Pa., assignors to Scott Paper Company, Philadelphia, Pa., a corporation of Pennsylvania No Drawing. Filed Oct. 8, 1965, Ser. No. 494,272 Int. Cl. DZlc 3/18, 3/20 US. Cl. 162-76 4 Claims ABSTRACT OF THE DISCLOSURE Process for pulping lignin-containing wood chips by impregnating wood chips with an oxidizing solution of an organic peracid and chlorine dioxide to elfect penetration into the wood chips of the oxidizing solution, removing the excess of oxidizing solution from said wood chips, digesting said wood chips by heating to accelerate oxidation of the lignin therein, immersing said wood chips in an alkali solution to solubilize oxidized constituents thereof, and mechanically defibering said chips.

This invention pertains to pulping of lignocellulosic materials contained in the stems of vascular plants, and, more specifically, to the pulping of wood whereby syngeristically coacting liquor components are used to defiber cellulosic fibers with heretofore unobtaiued yield. In particular, this invention pertains to pulping of softwoods and hardwoods by means of an organic peroxygen solution which has been modified by the addition of chlorine dioxide. The products of the process are within the scope of the invention.

Conventional wood pulping methods may be divided into two general classes: first, those dependent upon the reaction between lignin and sulfite solution in which the solubilization of lignin is accomplished by sulfonation and cleavage of the lignin molecule. The second class consists of the methods dependent upon reaction of lignin with alkali, preferably in the presence of sodium sulfide with solubilization resulting from highly polar groups of the lignin moiety so for-med.

In both classes of methods the chemical reactions in conventional pulping are carried out under rather drastic conditions of temperature, pressure and time. For example, in the conventional sulfite process the normal conditions are 140 C. to 160 C., 90 to 110 p.s.i. and 6 to 10 hours. For alkaline pulping (kraft), the normal conditions are 170 C. to 176 C., 100 to 135 psi and 2 to 4 hours.

The present invention may be better understood by referring to the major components of wood and the stems of other vascular plants. These components are cellulose, hemicellulose and lignin. Cellulose component of wood is most resistant to chemical attack. In general, this greater resistance is the basis of present pulping methods. In order to loosen and disintegrate the wood structure, the removal of lignin is normally essential. However, the lignin component, if it could be rendered useful in paper manufacture would give higher yields. The hemicellulose component of the wood is a polysacice charide and is a colorless material similar to cellulose except for being more swellable in water. It is acceptable as an ingredient of paper pulp and for some purposes it has superior paper-making properties. Unfortunately, the drastic conditions necessary for lignin removal by the conventional process result in hydrolysis and dissolution of the major part of hemicellulose. Thus, the present pulping reactions as now practiecd can not discriminate between removal of lignin, removal of polysaccharides and freeing of cellulose without dissolving lignin to any great extent. Hence, in full chemical pulping the removal of sufficient lignin to permit disintegration of the wood to pulp results in a yield of unbleached pulp in the range of only 45% to 50% of the original wood.

In Ser. No. 462,116, filed June 7, 1965, and assigned to the same assignee, there is disclosed a method for removing lignin under conditions that retain a major portion of the hemicellulose in the pulp as well as the major portion of cellulose to achieve a yield of unbleached pulp of from about to about based on the wood chips used. These yields have been achieved by' using an oxidizing agent such as peracetic acid to liberate the cellulose by lignin removal. A major portion of the lignin is believed to be solubilized and removed from cellulose by means of dilute alkali solution.

In the present process, the novel and surprising feature resides in the observation that exposure of wood, and especially softwood, to relatively small quantities of an oxidizing agent, such as organic peroxide compound in combination with a synergistically coacting chlorine dioxide, results in yields up to 89% based on wood and active oxygen consumption of less than that when either an organic peroxide or chlorine dioxide is used to defiber cellulose to a given and equal percent yield when employing either of the components by itself. Moreover, the novel results are achieved with reduced liquor-towood ratios.

It is not known what chemical reactions are responsible for the high yields, in what form the lignin is removed from the wood, nor in what form it is left incorporated in the fibers. However, pulp obtained by the novel process is a chemical pulp and will likewise. use an insignificant amount of energy to liberate the cellulose fibers (when compared to mechanical or semi-chemical pulp). Liberation of the pulp in the present process is accomplished by using a dilute alkali solution on .the treated wood.

An additional advantage of this process is a by-product which appears to be a dicarboxylic compound in nature, and is believed at this stage to be a muconic acid.

Still further, it has been found that the novel process allows large-scale utilization of diverse species of wood and concomitant recovery of lignin derivatives from these species in a form especially suitable for utilizing lignin.

Although it is not known whether the active oxygen components react toward the lignin synergistically or catalytically, it is believed that both actions may be involved to a certain degree. Whatever may be the reaction, the novelty and unobviousness of the present process .is predicated on the yields of pulp heretofor thought to be unobtainable and the low consumption of the active oxygen.

The novel process described above has been accomplished by impregnating wood chips of the sizes normally employed in commercial pulp processes such as from about 0.05 to about A with a suflicient amount of an aqueous solution containing an organic peroxygen compound and chlorine dioxide. Of the organic compounds, the peracids are preferred. These consist of performic, peracetic, perbutyric and perbenzoic. Of the acids, the peracetic is preferred because of its behavior and exhibited properties. In general, a liquor-to-wood ratio of wood on a bone-dry basis is of from about 0.5 to about 1 and up to about to l. A preferred liquor-to-wood (L/W) ratio is of from 3:1 to 5:1. In order to aid the penertation of the liquor into the wood, the chips are brought into contact with a solution under pressure. Thereafter excess liquor is removed to the extent that the liquor-to-wood ratio is reduced to about 1:1, the chips heated for about hour to about 1, or about 50 C. to 90 C., and washed with hot water. The washed chips are then treated with of from 1% to 5% sodium hydroxide solution at about 2:1 up to :1 liquor-to-wood ratios, but preferably at a ratio of from 2:1 to 5:1, for about /2 hour to about 1 /2 hours at a temperature from about 50 C. to about 125 C., preferably about 50 C. Thereafter, the digested chips are disintegrated in a distintegrator to obtain a pulp. Very little energy is needed to defiber the pulp and the recovered pulp is in the form of fibers of the type normally encountered in chemical pulping, although these fibers contain a considerable portion of lignin material. Because of this high utilization factor of wood, and the unexpected properties of the end product of this process, the products obtained are believed to be novel.

Peroxygen compounds which are useful in the present process are those that are water soluble and stable or capble of stabilization at up to 50 C., but preferably up to 100 C. and consist generally of alkyl hydroperoxides, alkylperoxides, acyl hydroperoxides, acyl peroxides, acyl alkylperoxides, hydroxy alkylhydroperoxides, hydroxy alkylperoxides, ozonides, oxozonides, and alkylidene peroxides. Of the above compounds, those of short carbon atom chains in the alkyl moiety are preferred and especially those which are stable at the process conditions and in the impregnation environment used, and most preferably, those that have a high percentage of active oxygen in reference to their molecular Weight. Of the above group, the acyl hydroperoxides such as the performic, peracetic, perbutyric and perpropionic, pervaleric, percaproic, etc., are the preferred subgroup of compounds. Of these, peracetic acid is the most preferred species.

In particular, the present process sequence is accomplished by the use of a solution of peracetic acid as the preferred species in conjunction with a chlorine dioxide 4 brought up is from about 30 C. to start and about 90 C. at the termination, but preferably at about C. at the termination of the digesting or oxidizing step when this temperature is maintained for about 15 minutes to about 120 minutes. A water washing step may follow the oxidizing step.

Thereafter, an alkaline solution such as a solution of sodium hydroxide, potassium hydroxide and the soluble alkaline earth hydroxides may be employed at a concentration of from about 1% to about 5%, but preferably below 3%, to remove the lignin derivative material, e.g., muconic acid and other solubilized inconstants. Sodium hydroxide solution is preferred.

Wood chips derived from softwoods (derived from some gymnosperms) and hardwoods (derived from some angiosperms) are useful in the present process. Of the softwoods, the coniferous species such as spruce, pine, hemlock, fir, cedar, etc., are the most desirable. Of the hardwoods, those useful in the present pulping process are alder, aspen, beech, oak, gum, birch, etc.

While, in general, the softwood species have consumed a greater percentage of active oxygen in a peracetic type of pulping, it has now been found that by use of the peracid solution in conjunction with chlorine dioxide, the amount of active oxygen used can be drastically reduced.

As noted before, for solubilizing the carboxylic acid derivative of lignin, sodium hydroxide solutions are most suitable. Ammonium hydroxide and other alkali hydroxides and their alkaline salts such as sodium carbonate, sodium carbonate, etc., are applicable, but the cost factor and efficiency are not as great. Other materials similarly suitable for solubilizing lignin may be used for their specific properties such as ammonium hydroxide, because of its ease of recovery, and other amines for their solubility.

In the examples to follow, all parts are by weight unless otherwise expressed. These examples are merely illustrative of the invention and are not intended to limit the broader scope of it.

Example 1 In Table I below, several pulp runs are illustrated using spruce wood in the form of chips. The first cook shows that when using no chlorine dioxide and effecting the cellulose impregnation without it, the total oxygen consumption and the yields are considerably lower when compared with cook No. 6. The same comparison applies when chlorine dioxide is used by itself in cook No. 2. This chlorine dioxide run also illustrates the inherent problems associated with chlorine dioxide solubility. In all the runs, a small amount of mechanical energy is needed to separate the fibers.

TABLE I.-DEFIBERING OF SPRUCE WOOD BY MEANS OF PERACETIC ACID, CHLORINE DIOXIDE AND A SYNERGISTIC MIXTURE OF CHLORINE DIOXIDE AND PERACE- TIC ACID Cook 1 Cook 2 Cook 3 Cook 4 Cook 5 Cook 6 Charge of chips, g 10 10 10 10 10 10 Concentration of cooking liquers before mixing:

C10 percent 0 1. 59 0. 7 0. 7 0. 7 O. 7 Peracetic acid, percent l4. 5 0 7. 0 7. 0 5. 0 7. 0 Total acttive oxygen concentration after mixing, 3.06 0.75 2.19 1. 94 1. 23 1.

percen Total active oxygen used on wood, percent 5. 8 6.2 2. 79 2. 69 2. 21 1.68 L/ W ratio durlng digestion l. 6 1.6 1.6 1.4:1 1. 05:1 0. :1 Y1e1d, percent on wood 75. 7 75. 0 78. 0 80. 0 89. 0 86. 0 Berakmg length of standard handsheets, M 7, 600 G, 000 5, 900 2, 000 2, 200

1 C10 is not soluble in concentrations of more than 1.5 under ordinary conditions. A one-stage impregnation followed by vapor phase digestion would only consume about 3% 0102. It was necessary to do two such treatments to get to the level of 6.2% active oxygen on the wood. This was done to get to the level of C102 necessary for defibering.

solution in an impregnating 20 C. to about 30 C. and at a pressure of from 1 atmosphere to about 20 atmosphere in the impregnating step. Nitrogen gas may be used as a pressure-exerting medium, although liquids introduced under pressure may likewise be employed in the digesting step. The temperature range at which the chips It is noted that the defibering mechanism is not necessarily synonymous with the delignification. The wood can be altered in such a manner that defibering can 'be effected without any appreciable loss in yields of long-fibered pulp. In other words, if it is assumed that lignin serves as a cross-linking network to reduce the swelling action of and the peracetic chlorine dioxide solution may be 75 water upon wood and to prevent disintegration of the wood structure in water or aqueous solutions, then the novel results herein reported indicate that certain oxidative reactions have the ability to sever this network at key junction points thus facilitating the disintegration of wood with relatively little removal of material.

Example 2 Chips of southern pincwere treated according to the described method and gave a pulp yield of 91% Liquor/wood ratio during digestion 0.80:1 Yield, percent on wood 91 Example 3 Table II below shows results obtained by using a synergistic combination of chlorine dioxide and peracetic acid to pulp beech chips by the two-stage process described above. Very low active-oxygen consumption is exhibited in conjunction with high yields. Defibering appears to occur by a mechanism other than complete delignification.

TABLE II.DEFIBg What is claimed is:

1. A process for pulping lignincontaining wood chips comprising the steps of:

(a) intimately contacting Wood chips with an aqueous solution of peracetic acid in an amount in the range from about 1% to about 15% and chlorine dioxide in an amount in the range between about 0.2% and 1.5% to effect penetration of said chips with said solution;

(b) removing the excess solution of peracetic acid and chlorine dioxide;

() heating the chips at a temperature in the range from about 50 C. to about 90 C. for a period in the range from hour to about 1 hour to digest the same;

((1) solubilizing oxidized constituents of said chips by immersing the same in a sodium hydroxide solution; and

(e) mechanically defibering the chips.

2. A process as set foith in claim 1 wherein the sodium hydroxide solution is present in a liquor-to-wood ratio in a range of about 2:1 to about 10:1.

3. A process as set forth in claim 1, wherein the liquor to-wood ratio. in step (a) is less than about :1, and wherein the liquor-to-wood ratio in step (c) is less than about 1.6:1.

4. A process as set forth in claim 3, wherein the liquor- RING OF BEECH CHIPS BY MEANS OF A SYNERGISTIC COMBINATION F CHLORINE DIOXIDE AND A PERACEIIC ACID Cook 1 Cook 2 Cook 3 Cook 4 Cook 5 Cook 6 Charge of chips, g 10 10 10 10 10 10 Concentration of cooking liquors before mlxing:

010 percent 0. 5 0. 4 0.3 0. 2 0. 2 O. 2 Peracetic acid, percent 5. 0 4. 0 3. 0 3. 0 1. 5 1. 0 Total ac tive oxygen concentration after mixing, 1. 44 1. 0.83 0.90 0. 488 0.295

11910611 it Total active oxygen used on wood, percent 2. 29 1. 88 1. 47 1. 50 0. 865 0. 534 Liquor/wood ratio during digestion 0. 9:1 0. 9:1 0. 9:1 0. 9:1 0. 9:1 0. 8:1 Yield, percent on wood 76.0 77.0 80. 0 81. 0 86. 0 88.0

The liquors in all of the above cases are black and upon being acidified give a brown precipitate and brown supernatant liquor. The precipitate may be obtained by using the washed liquor from the first stage. As noted above, the elfect of chlorine dioxideand peracetic acid results in an increase of pulp yield which is still obtained by a chemical defibering mechanism. The consumption of active oxygen to achieve a certain yield is reduced in both the hardwoods and softwoods.

Novel papers obtained by the present process have the tensile properties depicted in Table I as measured by TAPPI methods T 205 M-58 and T 404 08-61. The pulp produced by this process has tensile properties varying with the yield. A very strong sheet can be obtained if the yield is around 70% but a less strong sheet if the yield is 90%. This process has the obvious advantage of yielding pulps of variable strength depending on the operating conditions employed.

to-wood ratio in step (c) is in the range from about 0.8:1 to about 1.621.

References Cited S. LEON BASHORE, Primary Examiner US. Cl. X.R. 

